Halophosphate phosphor treatment process

ABSTRACT

THIS INVENTION RELATES TO A WASHING TREATMENT FOR A

United States Patent O 3,725,297 HALOPHOSPHATE PHOSPHOR TREATMENTPROCESS George R. Gillooly, Cleveland Heights, Ohio, assignor to GeneralElectric Company, Schenectady, NY. No Drawing. Filed Mar. 1, 1972, Ser.No. 231,024 Int. Cl. C09k 1/36 US. Cl. 252301.4 P Claims ABSTRACT OF THEDISCLOSURE This invention relates to a washing treatment for ahalophosphate type phosphor and more particularly to a washing processfor improving the luminescent output of alkaline earth halophosphatephosphors by removing any free antimony residue remaining after phosphorpreparation and with modifications of the same washing treatment beingfurther able to remove other residual impurities.

BACKGROUND OF THE INVENTION Alkaline earth halophosphate phosphors arethe luminescent materials most commonly used in fluorescent lamps today.In general, halophosphates are compounds more or less analogous to thenatural mineral apatite and can be represented by a formula such asWhere L represents a halogen or mixture of halogens and M and Mrepresent either diiferent or identical alkaline earth metals ormixtures of such metals. Such artificially produced compounds useful asluminescent materials or phosphors when suitably activated are disclosedin Pat. 2,48 8,733-McKeag et al. The most commonly used halophosphatephosphor is calcium halophosphate believed to have the followingformula:

Antimony is used as an activator to contribute a luminous emission bandin the blue region of the phosphor and manganese as a second activatorto contribute an emission band in the orange-red region; variousqualities or color temperatures are obtained by varying the proportionof these two activators. This phosphor is usually made by mixing CaHPOCaCO CaF MnCO and Sb O together as a dry mixture and firing this mixturein trays at a temperature in the range from 1000-1200 C. It has beenestablished that there are other than pure apatite structures presentafter the solid phase firing reaction of the phosphor. Some of thesematerials are antimony oxide, calcium pyrophosphate, calcium antimonateand manganese-containing compounds. When this phosphor is used inelectric discharge lamps, particularly in fluorescent lamps, thephosphor is subjected to a number of adverse conditions in lampmanufacture which reduce its response to ultraviolet exciting radiationand its reflectance and transmission of visible light. For instance, thepresence of any free antimony residue or antimony compound in thephosphor after preparation can result in poor lamp efiiciencies duringoperation of the lamp made with such phosphor. Likewise, the presence ofthe manganese and manganese compounds which are not part of the phosphorcrystalline structure can also deleteriously affect lamp performance.The free manganese oxides or manganese compounds may be adverselyaffected by oxiding conditions which can occur at some stages of lampmanufacturing and thereafter absorb both ultraviolet and visible lightto lower lamp operating efiiciency. If calcium is present in other thanthe apatite phosphor structure, there can also be a deleterous affectupon lamp performance and lumen maintenance.

A washing process for removing these residual materials after phosphorpreparation is disclosed in Patent 3,047,512 which uses an aqueoussolution of ethylenediamine tetraacetic acid. The disclosed method canbe carried out by agitating the phosphor in an ammoniacal solution ofethylenediarnine tetraacetic acid containing 3% or more by weight ofethylenediamine tetraacetic acid relative to the weight of phosphorbeing washed, thereafter separating phosphor from the liquid, andrinsing the phosphor in water to remove the residual ethylenediarninetetraacetic acid. This method represents an improvement over the otherknown washing methods which employ different acidic or basic materialsto solubilize the same residues for removal from the phosphor in that alesser concentration of the washing agent can be used. Moreparticularly, there is always a risk in washing the phosphor in thatsome of the dissolved residue as well as some portion of the Washingagent itself can be redeposited on the phosphor crystals after the finalwater rinse and thereby not completely achieve the desired objective.For this reason and the attendant cost considerations, it becomesdesirable to employ as little of the particular Washing agent selectedas is needed to remove the deleterious residual materials withoutreleasing either any residue or washing agent back into the phosphor orhaving to employ extra processing steps such as centrifuging to avoidthis result.

SUMMARY OF THE INVENTION Briefly, I have discovered that a significantimprovement in brightness, ultraviolet response and elficiency ofalkaline earth halophosphate phosphors can be achieved by washing themafter preparation with an aqueous solution containing a dissolved alkalimetal bitartrate compound to remove residual antimony which is notincorporated or firmly bound into the apatite structure of the phosphorcrystal lattice. It has further been found that favorable solubilitycharacteristics of the alkali metal bitartrate compounds in an aqueoussolution require approximately only stoichiometric amounts of thewashing agent with respect to the residual antimony content on thephosphor surface to eifect the removal by dissolution so that solutionsas low as 0.001 molar concentration may be used in place of the 5 molarpotassium hydroxide solution commonly employed as a conventional basewashing method. Moreover, the antimony product formed upon washing is asoluble salt not especially subject to hydrolysis with consequentprecipitation and redeposition on the phosphor during the washing orrinsing treatment. It has also been found that aqueous solutionscomprising a mixture of the dissolved alkali metal bitartrate compoundwith a correspondingly low concentration of ethylenediamine tetraaceticacid are effective to remove residual calcium and manganese from thephosphor along with residual antimony during the same washing treatment.

3 DESCRIPTION OF THE PREFERRED EMBODIMENTS In carrying out the processof the present invention to remove residual antimony fromantimony-activated halophosphate phosphors, the phosphor particles afterpreparation are contacted with an aqueous solution of an alkali metalbitartrate compound, the phosphor is thereafter separated from theaqueous solution and the washed phopshor is finally rinsed with water orsome other suitable liquid solvent to remove any residual alkali metalbitartrate compound. Weight ratios as low as 2% and less by weight ofthe alkali metal bitartrate compound relative to the weight of thephosphor have been found effective in removing the ordinary residuallevel of antimony not contained in the phosphor apatite structure whenthe phosphor has been prepared by conventional manufacturing techniques.The correct amount of a particular alkali metal bitartrate compound tobe employcd for this purpose need be not greater than the stoichiometricamount dictated by the following chemical equation:

wherein Me is an alkali metal ion with potassium bitartrate being apreferred washing agent by reason of the greater solubilityantimony-potassium tartrate (tartar emetic) than potassium bitartrate(cream of tartar) which favors completion of the above-specifiedreaction. Various modifications of the washing treatment above generallydefined can provide an even more efficient phosphor. By way of example,the incorporation of ethylenediamine tetraacetic acid in an aqueoussolution of the alkali metal bitartrate compound accomplishes removal ofexcess calcium and manganese along with residual antimony at similarminimum weight ratios for this sequestering agent relative to weight ofthe phosphor as was previously stated for the alkali metal bitartratecompound. It should also be possible to provide an aqueous solutioncomprising mixtures of an alkali metal bitartrate compound with stillother washing agents in lesser amounts to provide a more effective meansof removing residual calcium, manganese and antimony cations from thephosphor. Likewise, it is contemplated to prepare a solution of thealkali metal bitartrate compound during the washing treatment byreaction between a basic alkali metal compound such as alkali metalhydroxide and tartaric acid using stoichiometric properties of thestarting materials so as to produce the bitartrate or acid tartratereaction product in situ as distinct from the full tartrate compound.

The following exemplary description includes preferred methods forWashing a cool-white type phosphor having the chemical composition andphysical structure hereinbefore described with aqueous solutions of aselected alkali metal bitartrate compound or an aqueous solutioncontaining such alkali metal bitartrate compound with ethylenediaminetetraacetic acid and the results obtained thereby.

- Example 1 Six grams of potassium bitartrate were dissolved in oneliter of hot water, and 25 milliliters of the washing solution thusprepared was mixed with grams of the cool-white phosphor. The resultingslurry was heated to the boiling point with rapid stirring andthereafter filtered to remove the wash water containing dissolvedantimony from the phosphor particles. The washed phosphor crystals werethereafter rinsed in hot water to remove any residual potassiumbitartrate with the free antimony content of the phosphor being measuredbefore and after the washing treatment given. The particular phosphorwas found to contain 0.805 weight percent antimony before the washingtreatment with said antimony content being reduced to 0.783 weightpercent by washing. A brightness test performed by exposing the phosphorto a source of 2537 A. wavelength radiation 4 resulted in 98.7% relativebrightness for the unwashed phosphor which rose to 100.2% relativebrightness after the washing treatment.

Example 2 The same composition phosphor used in Example 1 was washedwith aqueous solutions containing ethylenediamine tetraacetic acid ormixtures of ethylenediamine tetraacetic acid with potassium bitartrate.The treated phosphors were then suspended in suitable vehicularsuspension system to provide a fluorescent lamp coating in a 40-wattfluorescent lamp and the lamp performance measurements compared forthese washing treatments.

The washing procedure employing a mixture of potassium bitartrate withethylenediamine tetraacetic acid was accomplished in the followingmanner. Fifteen grams of tartaric acid were added to approximately 10liters of hot deionized water with stirring. Thereafter a quantity of5.6 grams of potassium hydroxide pellets was dissolved in a smallquantity of hot deionized water and then added to the tartaric acidsolution already formed. Approximately 1500 grams of the sifted phosphorwas added to the freshly prepared potassium bitartrate solution withstirring and an additional quantity of 12:5 liters of hot deionizedwater was added to the solution while being maintained at approximatelyC. The slurry was held at said temperature for approximately 5 minutesafter which approximately 18 grams of ethylenediamine tetraacetic acidwas added to the slurry with stirring being continued for an additional45 minute time period. At the end of this washing period, the slurry wasfiltered to separate the phosphor from the washing liquid, and thewashed phosphor cake then rinsed with hot deionized water to remove anyresidual ethylenediamine tetraacetic acid and potassium bitartrate. Therinsed phosphor was then centrifuged and a lamp suspension formed in theconventional manner.

The washing procedure used to remove residual calcium, manganese andantimony from the same phosphor with ethylenediamine tetraacetic acidalone is described in the previously referenced 3,047,512 patent, henceneed not be repeated again in detail. In brief, an embodiment of saidprocess was carried out for comparison purposes whereby the same washingtreatment described immediately above which employed an aqueous solutionof potassium bitartrate prepared in situ with ethylenediaminetetraacetic acid was repeated without using any alkali metal bitartratecompound. The relative lamp performance of the 3000-lumen rated 40-wattfluorescent lamps of these different washing treatments is reported Itcan be noted from the above comparison that a washing treatment inaccordance with the present invention provides a significant lumen gainat initial lamp operation and after hours operation.

It will be apparent from the foregoing description that a washingtreatment has been provided to improve the luminescent output ofalkaline earth halophosphate phosphors and that various modificationscan be made in the preferred embodiments shown. Consequently, it isintended to limit the present invention only to the scope of thefollowing claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A process to remove residual antimony from antimony-activatedhalophosphate phosphors which comprises contacting the phosphorparticles with an aqueous solution of an alkali metal bitartrate,separating the phosphor from the aqueous solution, and rinsing thephosphor to remove residual alkali metal bitartrate and antimonycompounds.

2. A process as in claim 1 wherein the aqueous solution comprises amixture of alkali metal bitartrate with another washing agent.

3. A process as in claim 2 wherein the washing agent is ethylenediaminetetraacetic acid.

4. A process as in claim 1 wherein the alkali metal bitartrate isproduced in the aqueous solution by reaction between a basic alkalimetal compound and tartaric acid.

5. A process as in claim 1 wherein the alkali metal bitartrate ispotassium bitartrate.

References Cited UNITED STATES PATENTS 3,047,512 7/1962 Martyny252-301.4 P 3,060,129 10/1962 Hoeikstra et a1. 252-3014 P 3,384,5985/1968 Friedman et a1. 252-301.4 P 3,485,767 12/1969 Ogrinc 252-3014 P3,538,013 11/1970 Graif 252-3014 P OTHER REFERENCES Wanmaker et al.ThePreparation of Calcium Haloph0sphate--Phi1ips Research Reports, vol. 10,pp. 11-38, February 1955.

ROBERT, D. EDMONDS, Primary 'Examiner

